Biomedical Engineering Reference
In-Depth Information
A
Position 1
n
= 1.000
n
′
= 1.333
FP of eye
F
′
of lens
B
Position 2
n
= 1.000
n
′
= 1.333
FP of eye and
F
′
of lens
Figure 7-9.
The effective power of a spectacle lens used to correct myopia is increased
by moving the lens toward the eye [from position 1 in (
A
) to position 2 in (
B
)].
The principles of lens effectivity also apply to hyperopia.
Consider the 2.82 D
hyperope who we discussed earlier in the chapter. This hyperopic patient requires
a corrective lens of
+
2.82 D in the plane of the cornea (i.e., a contact lens whose
power is
2.82D). What power spectacle lens is required to correct this refractive
error? Assume a vertex distance of 15.00 mm.
+
The secondary focal point of the corrective lens, whether it is a contact lens or a
spectacle lens, must be coincident with the far point of the eye (which is 35.46 cm to
the right of the cornea). From Figure 7-10, we see that the secondary focal length
of the spectacle lens must be 36.96 cm (35.46
+
1.50 cm
=
36.96 cm). The power of
this spectacle lens is
n
′
F
=
′
f
(100)(1.00)
F
=
36.96 m
F
=
+
2.71 D
